For a given optical configuration (channel, PFOV and Filter or CVF
), using figures 
to , find the
following data:
= ordinate of the solid line for the filter or CVF at
= ordinate of the dashed line for the filter (if shown)
= ordinate of the triangle labelled with the filter
number.
These are respectively the limiting fluxes achievable with S/N =
10 (with 3 records of 60 seconds (SW) or 20 records of 10 seconds
(LW)), taking into account only the HF noise, the flat-field noise
(
= 1%), or the dark noise (
= 0.5
ADU (SW) or 0.3 ADU (LW) rms, with gain 
2).
Taking 
(in mJy for a point source, or in mJy/
for an extended source) and S/N the expected flux of
the source and the needed S/N ratio of the observation, compute:
If 
this observation should be easy
to do, you can choose the parameter of the figure, or even try to
scale down 
, but not much since the minimum
number of exposures is 4. You could possibly shorten the
observation time by taking a shorter integration time, then 
increases roughly like 
.
If 
, increasing helps
only as long as 
, since the
S/N increases very slowly with as soon as flat-field
or dark noise dominates.
If 
or 
, this observation
needs respectively a dark or a flat-field of accuracy better than
the standard values. You can undertake this or these measurements,
but at a considerable observation time expense. Using the beam-
switching or the micro-scanning technique is then strongly
recommended.
In any case, must be larger than 4 for deglitching.